The present invention relates to electrophotography and more particularly, to a dry process developing method in electrophotography in which disadvantages inherent in the conventional dry process developing methods, for example, two component developing method employing non-magnetizable toner particles and magnetizable carrier material, and one component developing method employing magnetizable toner particles are eliminated.
Generally, two component developing methods such as the cascade developing method, magnetic brush developing method, etc., are well known in the art and have been put into practical application. In such developing methods, electrically insulating non-magnetizable toner particles having average particle diameters of 10 to 15 .mu.m and particles commonly known as carriers are mixed for use. In the cascade developing method, the non-magnetizable toner particles are charged through rubbing against the electrically insulating carrier particles of bead-like shape to be attracted onto surfaces of the carrier particles and transported to a developing position of a developing apparatus, while in the magnetic brush developing method, the carrier is formed into magnetizable particles mainly of iron of approximately 75 .mu.m in diameter to be magnetically attracted in the form of magnetic brush bristles onto an outer cylinder or sleeve of a developing apparatus in a known manner. In the magnetic brush developing method, the non-magnetizable toner particles are charged through friction against the carrier to adhere to the surfaces of the carrier particles and transported to the developing position in the similar manner as in the cascade developing method mentioned earlier. The electrically conductive carrier particles also serve as a developing electrode positioned extremely close to a photosensitive member during developing.
The dry process two component developing method as described above, however, has various problems particularly related to the carrier in that such carrier only serving in charging and transporting the non-magnetizable toner particles or as the developing electrode (in the case of the magnetic brush developing method) without directly engaging in the developing itself is not consumed at each time of copying, and thus gradually deteriorates with the increase of the number of copies taken, generally making it necessary to be replaced after a predetermined number of copies has been taken. Although the life of the carrier seems to have been prolonged to a considerable extent due to recent development of carriers having various coatings, replacement thereof is still required after use for a predetermined period of time. Furthermore, since the mixing ratio of the carrier to the non-magnetizable toner largely affects the quality of copied images, giving rise to adhesion of the carrier to the photosensitive member in some cases, stabilization of the mixing ratio i.e., replenishing of the non-magnetizable toner at a constant rate is required. Although various improvements have conventionally been proposed for such stabilization of the mixing ratio to be put into practical application, the constant rate replenishment is still difficult, with the developing apparatus tending to be undesirably large in size. Particularly, when the particle size of the carrier is too small or the mixing ratio is deviated to the carrier side, the carrier may adhere onto the surface of the photosensitive member in some cases, thus adversely affecting the quality of the copied images. Moreover, since the diameter of the carrier particle can not be made excessively small due to the above fact, increase of the surface area of such carrier particle is inevitably limited, and depending on the mixing ratio of the carrier to the non-magnetizable toner, there are cases where uneven charging for the non-magnetizable toner may take place.
For eliminating the inconveniences as described above, there has also conventionally been proposed a magnetic brush developing method, for example, in Japanese Patent Publication Tokkaisho 52/65443, wherein high resistance toner particles and magnetizable low resistance toner particles included in the developing material are subjected to triboelectric charging so that the high resistance toner particles are charged with polarity opposite to that of the electrostatic latent image, while the low resistance toner particles are charged with the same polarity as that of the electrostatic latent image, and by a magnet member which attracts the magnetizable low resistance toner particles through magnetic force weaker than the electrostatic attraction produced between the high and low resistance toner particles as a result of said triboelectric charging, the developing material is maintained in the form of magnetic brush which contacts the surface of the electrostatic latent image for developing such latent image into a visible toner image. The known method as described above, however, is not perfectly free from the drawbacks as described earlier, still having some problems to be solved.
In order to overcome the disadvantages inherent in the two component developing method as described above, there has conventionally been proposed one component developing methods employing magnetizable toner particles and the like, for example, in Japanese Patent Publication Tokkaisho 51/26046 in which only magnetizable toner particles having magnetizable fine particles exposed to surfaces thereof are employed for developing an electrostatic latent image of negative polarity formed on a photosensitive member of zinc oxide, and also in Japanese Patent Publication Tokkaisho 51/126836 in which there is employed a developing material constituted by attracting electrically conductive fine particles of approximately 0.01 to 2 weight% onto surfaces of particles which are a mixture of magnetizable fine particles and thermoplastic resin. Some of such conventional one component developing methods have already been put into actual use with direct type copying apparatuses, i.e., copying apparatuses which use photosensitive paper applied with photosensitive material without effecting transfer. Meanwhile, various other attempts have also been made to apply the one component developing method to the copying apparatuses of transfer type, but in such prior art as described above, there are difficult problems to be solved related to physical properties in the developing and transfer in that conditions contrary to each other i.e., electrical conductivity during developing and electrical insulation during transfer are simultaneously required. More specifically, while the developing is successful in the case of the electrically conductive, magnetizable toner particles having high electrical conductivity, there is a disadvantage such that during electric field transfer onto plain copy paper, the polarity of the magnetizable toner is varied due to injection of charge thereto from the copy paper, thus resulting in the phenomenon so-called "Blow-off" in which the toner once transferred onto the copy paper again leaves the same copy paper to cause non-uniform density and fogging or turbulence in the copied images.
For eliminating the undesirable non-uniform density and fogging or turbulence in the copied images as described above, there have conventionally been proposed various arrangements such as employment of electrically insulated copy paper (disclosed, for example, in Japanese Patent Publication Tokkaisho 50/117435), pre-heating of copy paper (disclosed, for example, in Japanese Patent Publication Tokkaisho 50/43936), and uniform exposure of photosensitive surface to light before or during transfer (disclosed, for example, in Japanese Patent Publications Tokkaisho 51/26044 and Tokkaisho 51/96332), etc., none of which is, however, related to improvement of the one component developing method.
On the contrary, the electrically insulating magnetizable toner particles have problems related to developing. More specifically, since such electrically insulating magnetizable toner particles are not sufficiently stable in charging, the developed images tend to be undesirably soiled, and for eliminating such disadvantages, auxiliary means, for example, means for subjecting the electrically insulating magnetizable toner to corona charging within the developing apparatus is required as disclosed in Japanese Patent Publication Tokkaisho 50/117432, thus resulting in complication in the structure of the developing apparatus.
Although a developing method employing magnetizable toner having properties intermediate the electrically conductive magnetizable toner and the electrically insulating magnetizable toner has also been conventionally proposed, for example, in Japanese Patent Publication Tokkaisho 50/92137, it is quite doubtful whether such magnetizable toner can satisfactorily provide the properties of the electrically conductive magnetizable toner and electrically insulating magnetizable toner, while problems arise in such magnetizable toner from the viewpoints of difficulty in manufacturing thereof, stability under various temperatures and humidity conditions, etc.
Another disadvantage inherent in all one component magnetizable toner particles is such that due to the necessity for employing magnetizable material, cost of the toner tends to be high, and that since it is hard to increase the ratio of resin in such magnetizable toner particles, there is a difficulty in fixing thereof.